1. Field of the Invention
This invention relates to a method for preparing an alkylhalosilane by direct synthesis using metallic silicon powder and alkyl halide, and more particularly to a method for preparing an alkylhalosilane of quality at a high formation rate and a high selectivity and in high yields.
2. Prior Art
With respect to the synthesis of alkylhalosilanes, Rochow first disclosed in U.S. Pat. No. 2,380,995 direct synthesis reaction between metallic silicon and an alkyl halide in the presence of a copper catalyst. Since then, there have been reported a number of research works relating to various co-catalysts used together with copper catalysts, various copper catalysts and treatment thereof, reactors, additives used during reaction, and the like.
The direct synthesis process involves activating a mixture (often referred to as a contact mass) comprising metallic silicon, a copper catalyst and a co-catalyst, and introducing an alkyl halide into the activated contact medium for accomplishing gas-solid direct contact between metallic silicon and alkyl halide, thereby producing alkylhalosilanes. A fluidized bed reactor is generally used in industrial practice. If a uniform fluidized state is not established within the reactor, the formation rate of alkylhalosilanes will have a profile within the reactor and the rate of heat removal from within the system ceases to be uniform within the reactor. The loss of uniformity often causes the catalyst particles to be sintered and segregate from the contact mass and hot spots above the average temperature in the reactor to develop, giving rise to many problems including deactivation of the catalyst, decomposition of the alkyl halide and the alkylhalosilanes formed, impurity carbon build-up, a selectivity decline and a yield decline. Because of these problems, long-term continuous operation becomes difficult. Local deceleration of alkylhalosilane formation in the reactor undesirably results in a lowering of the overall formation rate of alkylhalosilanes.
For establishing a uniformly fluidized state, the particle diameter and particle size distribution of the contact mass powder are important factors. In general, a contact mass having a smaller mean particle diameter tends to invite a poorly fluidized state because particle agglomeration and a channeling phenomenon are likely to occur. For example, Kunii and Levenspiel, "Fluidization Engineering," Second Edition, 1991, describes that particles having a diameter of less than about 30 .mu.m, divided into a particle group designated "Geldart C," are difficult to fluidize because of a strong force between particles.
It was believed that the metallic silicon powder in the reactor should preferably have a mean particle diameter of greater than 30 .mu.m in order to establish a relatively good fluidized state. JP-A 202892/1990 corresponding to U.S. Pat. No. 5,015,751 discloses that the metallic silicon powder used in the synthesis of alkylhalosilanes should preferably have a mean particle diameter of up to 1,000 .mu.m, especially up to 500 .mu.m and that best results are obtained when the metallic silicon powder used has a mean particle diameter of 100 to 150 .mu.m and a particle size distribution of 30 to 300 .mu.m. Also, JP-B 5396/1991 corresponding to (U.S. Pat. No. 4,554,370) discloses that the silicon used in the fluidized bed should preferably have a particle size of up to 700 .mu.m, a mean particle size of 20 to 300 .mu.m, and a mean diameter in the range of 100 to 150 .mu.m.
However, from the reaction point of view, as opposed to the establishing of a well fluidized state, it is usually preferred that the metallic silicon powder have a smaller particle diameter. The reason is that a smaller particle diameter ensures rapid and uniform reaction on account of the effective heat transfer between particles and the greater surface area of particles participating in the reaction. From this point of view, JP-A 188258/1995 corresponding to U.S. Pat. No. 5,312,948 discloses that the metallic silicon powder used in the direct synthesis reaction should preferably have a particle diameter in the range of 1 to 85 .mu.m. Allegedly, strictly maintaining the metallic silicon powder within the above particle diameter range improves the selectivity of dialkyldihalosilane and the yield thereof from the starting reactant without detracting from fluidity. However, since particles having a diameter of less than 1 .mu.m falling outside that range have a very large reactive surface area, the exclusion of these particles significantly reduces the reactive surface area, resulting in a drop of reactivity.
Therefore, it is an industrially important task to those skilled in the art that a metallic silicon powder satisfying both reactivity and fluidity can be utilized in the preparation of alkylhalosilanes.